Title: Retroactive
Author: loopy777
Rating: T
Genre: Suspense/Adventure
Status: Complete
Fandom: Avatar: The Last Airbender
Pairings: none
Summary: "A mix of adventure, mystery, and psychological horror, featuring Suki and Azula as they explore their shared past and find that they have more enemies in the new post-war world than either could have expected. What is the nature of the conspiracy, who is in on it, and who will survive the resulting apocalypse? Based on an idea by Lavanya Six."
Link: https://www.fanfiction.net/s/8280375/1/Retroactive

Okay, this fic is amazing. I read it the first time around as it was coming out. Loopy777 is the author of the (somewhat-notorious) Mai's Ramblings series, and he's a very effective humour writer, but his suspense stuff is equally excellent, if not better. The plot is tied tightly up and it begins to unravel tantalisingly chapter-by-chapter - and it's really fascinating to read. A warning, though: I haven't read it in a while, so take what I say with a grain of salt.